Smart overlay control

ABSTRACT

An automatic method to maintain and correct overlay in the fabrication of integrated circuits is described. An overlay control table is automatically generated for lots run through a process tool. An overlay correction is calculated from the overlay control table and sent to the process tool for real-time or manual overlay correction.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

The present invention relates to the fabrication of integrated circuit devices, and more particularly, to a system of automatic overlay correction in the fabrication of integrated circuit devices.

(2) Description of the Prior Art

Lithography is widely used in integrated circuit fabrication. A material layer is deposited on a wafer. A masking material is formed over the material layer and exposed in a particular pattern to form a mask for etching, for example. The material layer is then etched where it is not covered by the mask. Another material layer is deposited over the patterned first material layer and the process is repeated to pattern the second layer. This process is repeated many times in the fabrication of an integrated circuit device. It is important that the patterns are aligned properly between layers. Alignment marks are used to align each layer to the previous layer. For example, a “box-in-box” alignment mark in a scribe line may be used. The previous layer alignment mark box is larger than the current layer's box. The centers of each box are measured in both the x- and y-directions. The difference in the box centers is the translation error of one layer to the other.

Typically, the engineer will manually feed back the translation to the exposure tool, perhaps weekly or monthly for correction of the overlay; that is, how closely two layers are aligned. This manual process is not a real-time correction process. With this method, alignment performance is unstable. The exposure tool may drift at any time. Different layers may have different topology or the wafer may warp slightly due to prior annealing, and so on. Many factors may affect alignment. It is desired to apply overlay correction in a real-time manner.

U.S. Pat. No. 6,218,200 B1 to Chen et al shows a method of aligning multiple layers in an automatic overlay correction system. U.S. Pat. No. 6,127,075 to Hsu teaches a method for checking the accuracy of a measuring instrument used to measure overlay. U.S. Pat. No. 6,023,338 to Bareket discloses a novel alignment mark pattern and measuring method.

SUMMARY OF THE INVENTION

Accordingly, it is a primary object of the invention to provide an automatic method to maintain and correct overlay in the fabrication of integrated circuits.

It is a further object of the invention to automatically generate an overlay control table for real-time overlay control.

Another object is to automatically generate an overlay control table for manual overlay control.

In accordance with the objects of the invention, a smart overlay control system is achieved. The overlay control system comprises a plurality of process machines, a data collection tool which measures overlay error from a run of a lot through a process machine, an overlay tool servo which processes overlay error data, sends overlay error data to an operator for use in manual overlay correction, and sends overlay error data to a smart overlay control servo for use in automatic overlay correction, a smart overlay control servo which generates an overlay control table for each lot type, and an equipment servo which obtains previous layer correction and fine tuning values from the smart overlay control servo for a lot on a process machine, calculates an overlay correction using the previous layer correction and the fine tuning values, and sends the overlay correction to the process machine.

Also in accordance with the objects of the invention, an automatic method to maintain and correct overlay in the fabrication of integrated circuits is achieved. An overlay control table is automatically generated for lots run through a process tool. An overlay correction is calculated from the overlay control table and sent to the process tool for real-time or manual overlay correction.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings forming a material part of this description, there is shown:

FIG. 1 illustrates a flow chart of the smart overlay control (SOC) system of the present invention.

FIG. 2 illustrates a flow chart of a preferred method of using the SOC system of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The process of the present invention provides a smart overlay control (SOC) system that provides real-time overlay feedback and correction. The system also provides a solution for a foundry fabrication mixing run. That is, lots of various types and needing various overlay corrections can be mixed together in this system.

FIG. 1 is a flow chart of the smart overlay control (SOC) system of the present invention. Lots of wafers to be processed enter the system at 10. The In-line Equipment Servo 14 determines the products' running condition; i.e. overlay correction, exposure energy, focus offset, etc. This Servo 14 provides input to the process machine 16 which is the exposure tool to be used in the lithography process.

Box 20 represents the data collection feature of the SOC system. The Overlay Measurement Tool 22 takes measurements of the alignment marks from layer to layer. The Overlay Analysis Servo 24 models raw data from the Overlay Measurement Tool 22. The output of the data collection box 20 goes to the data processing box 30.

The Overlay Tool Servo 32 filters abnormal data and sends measurement data to the Smart Overlay Control Servo 40 and to lot tracking systems, such as PROMIS.

The Smart Overlay Control Servo 40 automatically generates an overlay control table. Tables are generated for each lot type. Each table encompasses a given technology, layer, and exposure tool combination. The current layer overlay condition C_(n) is calculated from the previous layer overlay correction plus fine tune adjustment.

Table 1 illustrates the Smart Overlay Control Tuning Algorithm. TABLE 1 Tool Overlay Residual Error Fine Tune Lot C_(n) = P_(n) + F_(n−1) O_(n) R_(n) F_(n) n = 1 C₁ = P₁ + K O₁ R₁ = K − O₁ F₁ = R₁ n = 2 C₂ = P₂ + F₁ O₂ R₂ = F₁ − O₂ F₂ = ½ * (R₁ + R₂) n = 3 C₃ = P₃ + F₂ O₃ R₃ = F₂ − O₃ F₃ = ⅓ * (R₁ + R₂ + R₃) n = 4 C₄ = P₄ + F₃ O₄ R₄ = F₃ − O₄ F₄ = ⅓ * (R₂ + R₃ + R₄) n = 5 C₅ = P₅ + F₄ O₅ R₅ = F₄ − O₅ F₅ = ⅓ * (R₃ + R₄ + R₅) n = 6 C₆ = P₆ + F₅ O₆ R₆ = F₅ − O₆ F₆ = ⅓ * (R₄ + R₅ + R₆) n = k C_(k) = P_(k) + F_(k−1) O_(k) R_(k) = F_(k−1) − O_(k) F_(k) = ⅓ * (R_(k−2) + R_(k−1) + R_(k))    n = k + 1 C_(k+1) = P_(k+1) + F_(k)  O_(k+1) R_(k+1) = F_(k) − O_(k+1) F_(k+1) = ⅓ * (R_(k−1) + R_(k) + R_(k+1)) where C_(n) is the current layer overlay correction

-   -   P_(n) is the previous layer overlay correction; P₁=0.     -   O_(n) is the overlay measurement error     -   R_(n) is the residual overlay error

F_(n) is the SOC fine tune. Initially, this is the first residual error, then the average of the residuals of prior lots and the current lot. A running average of the residual errors of the last two lots plus the current lot is used.

K is the average process offset. This is supplied with the initial data for the first lot and may be zero.

The P_(n) values come from the Lot Information Servo 34. For manual overlay correction, the overlay correction table is sent to the Lot Information Servo. For automatic overlay correction, the overlay correction table is sent to the Smart Overlay Control Servo 40.

The In-Line Equipment Servo 14 queries the Smart Overlay Control Servo 40 for the previous layer correction and fine tuning values P_(n) and F_(n), respectively. The In-Line Equipment Servo calculates the correction C_(n) from the P_(n) sent to it and the previous F_(n-1). The In-Line Equipment Servo 14 sends the overlay correction to the Process Machine 16. The overlay correction is then sent from the Process Machine 16 to the Smart Overlay Control Servo 40 to be used as the next stage's P_(n) for that lot.

The process of the present invention can be used with a single alignment mark system or with a multiple alignment mark system. In the case of the multiple alignment mark system, the weight of P_(n) is set to zero; that is the previous layer overlay correction is not used. This is because in a multiple alignment mask system, the alignment mask is exposed and the layer patterned at the same time for some critical layers.

FIG. 2 is a flow chart illustrating the processing of a lot through a process machine using the Smart Overlay Control system of the present invention. If this is the first lot of its type to be run through the process machine 16 (in FIG. 1), an Overlay Control Table is generated for the lot (step 103) where P1 is set to 0. Now, in step 105, the previous layer correction and fine tuning values are obtained by the In-Line Equipment Servo 14 from the Smart Overlay Control Servo 40. The Overlay Control Servo 40 has obtained the Overlay Control Table from the Overlay Tool Servo 32. Now, in step 107, the In-Line Equipment Servo 14 calculates the overlay correction Cn and sends it to the Process Machine 16.

The lot is run on the Process Machine 16 (step 109). The data collection system 20 measures the alignment marks and sends the data to the Overlay Tool Servo 32 for updating the Overlay Control Table (step 111).

The Smart Overlay Control system of the present invention improves cycle time and decreases process engineers' loading. The SOC system has been implemented and has been shown to improve the mean plus 3 sigma overlay performance measurement by more than 25%. The rate of rework due to misalignment was found to be improved from more than about 0.5% by lot to less than 0.3% by lot.

While the invention has been particularly shown and described with reference to the preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made without departing from the spirit and scope of the invention. 

1. An overlay control system comprising: a plurality of process machines; a data collection tool wherein said data collection tool measures overlay error from a run of a lot through a process machine; an overlay tool servo wherein said overlay tool servo processes overlay error data, sends said overlay error data to an operator for use in manual overlay correction, and sends said overlay error data to a smart overlay control servo for use in automatic overlay correction; said smart overlay control servo wherein said smart overlay control servo generates an overlay control table for each lot type; and an equipment servo wherein said equipment servo obtains previous layer correction and fine tuning values from said smart overlay control servo for a lot on a process machine, calculates an overlay correction using said previous layer correction and said fine tuning values, and sends said overlay correction to said process machine.
 2. The system according to claim 1 wherein said overlay correction for a lot's current layer on said process tool is equal to an overlay correction for said lot's previous layer on said process tool plus fine tuning wherein said fine tuning is equal to a running average of previous lots' overlay errors.
 3. The method according to claim 1 wherein said overlay control table encompasses a given technology, a particular layer, and a particular process tool combination.
 4. The system according to claim 1 wherein said overlay control is used with a single alignment mark system.
 5. The system according to claim 2 wherein said overlay control is used with a multiple alignment mark system and wherein said overlay correction for said lot's previous layer on said process tool is set to zero.
 6. A method of overlay control comprising: automatically generating an overlay control table for lots run through a process tool; and sending an overlay correction calculated from said overlay control table to said process tool in real-time.
 7. The method according to claim 6 wherein said overlay correction for a lot's current layer on said process tool is equal to an overlay correction for said lot's previous layer on said process tool plus fine tuning wherein said fine tuning is equal to a running average of previous lots' overlay errors.
 8. The method according to claim 6 wherein said overlay control table encompasses a given technology, a particular layer, and a particular process tool combination.
 9. The method according to claim 6 wherein said overlay control is used with a single alignment mark system.
 10. The method according to claim 7 wherein said overlay control is used with a multiple alignment mark system and wherein said overlay correction for said lot's previous layer on said process tool is set to zero.
 11. A method of overlay control comprising: automatically generating an overlay control table for lots run through a process tool; and sending an overlay correction calculated from said overlay control table to be used in a manual or real-time overlay correction process.
 12. The method according to claim 11 wherein said overlay correction for a lot's current layer on said process tool is equal to an overlay correction for said lot's previous layer on said process tool plus fine tuning wherein said fine tuning is equal to a running average of previous lots' overlay errors.
 13. The method according to claim 11 wherein said overlay control table encompasses a given technology, a particular layer, and a particular process tool combination.
 14. The method according to claim 11 wherein said overlay control is used with a single alignment mark system.
 15. The method according to claim 12 wherein said overlay control is used with a multiple alignment mark system and wherein said overlay correction for said lot's previous layer on said process tool is set to zero.
 16. A method of overlay control comprising: measuring overlay error after use of an exposure tool; automatically generating an overlay control table for lots run through said exposure tool using said measured overlay error and a previous layer for each of said lots; and sending an overlay correction calculated from said overlay control table to be used in a manual or real-time overlay correction process.
 17. The method according to claim 16 wherein said overlay correction for a lot's current layer on said exposure tool is equal to an overlay correction for said lot's previous layer on said exposure tool plus fine tuning wherein said fine tuning is equal to a running average of previous lots' said overlay errors.
 18. The method according to claim 16 wherein said overlay control table encompasses a given technology, a particular layer, and a particular exposure tool combination.
 19. The method according to claim 16 wherein said overlay control is used with a single alignment mark system.
 20. The method according to claim 17 wherein said overlay control is used with a multiple alignment mark system and wherein said overlay correction for said lot's previous layer on said exposure tool is set to zero. 